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Comment by joe_the_user | Hacker News Reader

I can't see how mid-drives are significantly superior. An electric motor doesn't need any gear advantage and sending more force to the rear wheel seems like it's actually a disadvantage since it put more stress on bicycle components that are often designed for just human levels of force.

As far as I can tell, the mid-drives became dominant just 'cause people wanted to have something the looked more "seamless".

I have rear drive ebike (Avanton Pace 300 v1) and I'm very happy with it (it was also cheap as a show room model). It's responsive and I can pedal up any hill to about a 25% grade. The controller is still mid-frame and the battery inset into the down tube. Thinking about it, my ideal ebike would be a heavy duty frame with all the components just bolting on (plus could use power tool batteries).

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In the EU, e-bikes are limited to a maximum power output of 250 watts - anything over that is a motorbike. That's basically the same power output as a decently fit cyclist. A mid-drive allows for sustained high torque even with limited power. Water ingress issues aside, mid-drives also tend to be more durable in off-road applications where the rear wheel is subject to severe shock and vibration.

As I said, my rear drive gives what seems like high torque to me (I think bike's at about the power you mention). I've only taken my bike off-road a bit but it seems like the force on the rear hub and the bottom bracket are things that simply have to be designed for. The bottom bracket experiences both frame flex and the rider pounding the pedals, so it's subject to punishment and the rear hub is cushioned by the tires, spokes and any rear suspension present.

Power is power, there is no combined "power + torque" that measures how much wind you have in the sails. Torque just characterises how much power you can get along different RPMs.

But the EU/UK 250 W cap is so low that it seems unlikely torque characteristics would enter the picture significantly unless you're going very slowly. I think most of the mainstream EU ebike motors are capped this way (vs designed to have a natural peak output of 250W).

Torque (at the wheel) does matter as well as power. With a hub motor, the gear ratio is typically fixed so you're at the mercy of the motor characteristics, but driven through the chain, you can change it. I used to have a low powered hub motor e-bike and it was fast but pretty useless at accelerating from stopped or climbing hills. At these very low speeds, an optional low gear would have put the motor in a higher power point on its speed-torque curve.

Though perhaps you're saying these motors are so over-rated that even close to stall torque, their reduced power output is still 250W? That would be an efficiency loss but not a power loss since they're not allowed to deliver full power at higher speeds.

>> Power is power, there is no combined "power + torque" that measures how much wind you have in the sails. Torque just characterises how much power you can get along different RPMs.

> Torque (at the wheel) does matter as well as power.

If you are getting 250 W of power at the wheel (in other units, 250 Joules per second, or 250 Newton-meters), that's what you are getting, no torque about it.

>> the EU/UK 250 W cap is so low that it seems unlikely torque characteristics would enter the picture significantly unless you're going very slowly

> Though perhaps you're saying these motors are so over-rated that even close to stall torque, their reduced power output is still 250W?

Yes, I was saying something close to that ("very slowly").

IOW the overprovisioning of the motor combined with the power cap makes the motor work much more like a "always 250 W" power source than a "250 W peak output modified by torque" power source.

One nit, it's max /continuous/ output of 250W. Bikes with a temporary boost function to 500W are still legal and they're great to ride.

Electric motors have a wide power band but it isn't infinite[1], gearing does make a difference. If I'm in a higher gear on an incline my motor has significantly less usable torque vs shifting down to a gear that's closer to the power band.

Now if that is worth the complexity/cost is a fair debate but there are mechanical advantages to mid-drive motors.

[1] https://forums.electricbikereview.com/threads/ebike-motor-po...

One might expect some disadvantages to having more spinning mass in the rear wheel.

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